This invention relates to direction finding (DF) of remote radio frequency (RF) emitters using phase measurements made between pairs of antennas. More particularly, the invention relates to apparatus for measuring the direction of arrival (DOA) of a RF planer wave front utilizing phase bias error developed between elliptically polarized antennas which have non-parallel boresights, or which are "squinted".
The prior art teaches that there are two known methods of emitter angular location. These are commonly referred to as the amplitude DF and phase interferometer DF techniques.
A conventional approach for amplitude direction finding is to use pairs of antennas whose boresights are squinted, or offset, at an angle from each other. The system measures the relative power received by the two antennas, and, by knowing the antenna amplitude response, determines the emitter's angular location. Since the amplitude response varies with emitter polarization, and amplitude comparison accuracy is a strong function of the signal to noise power ratio (SNR), this technique is intrinsically inaccurate. It is generally not suitable for generating precision angle measurements required in bearings-only passive location. However, an advantage of amplitude systems of this type is that the squinted antenna pair may be located at any distance from one another, e.g., on different wingtips of an airframe. Because of this simplicity, amplitude DF systems are typically used on many aircraft as part of the radar warning system (RWR).
By contrast, interferometer systems measuring phase rather than amplitude between pairs of antennas generate emitter DF more accurately than amplitude systems, but these systems are more complex. In an interferometer, the antenna pair placement must be very precise to resolve the angle of arrival (AOA) ambiguities. Also, multiple antenna pairs are typically required to perform this ambiguity resolution.
In commonly assigned U.S. Pat. No. 5,343,212 there is described a passive emitter location technique that can use the phase measured across RWR amplitude DF antenna pairs. Thus, this technique obtains some of the advantage of an interferometer system, but can utilize elements of existing amplitude systems without requiring multiple antennas. However, this method uses signal processing techniques based on partitioning the emitter azimuth from the observer to the radar horizon. This partitioning results in many estimators that must be implemented in software. When the aircraft is at significant altitude above ground, the number of partition enhanced estimators required can be very large.
It is therefore an object of the invention to provide apparatus for generating AOA or DOA information which takes advantage of the relative simplicity of an amplitude DF system having squinted antennas but in fact uses the phase bias induced by the squinting antennas.
Another object of the invention is to provide apparatus capable of making such measurements without requiting emitter frequency measurements.